A joint for a drill tube transfers the torque power from one tube part to another. Because of a thinner wall thickness, the joint is generally weaker than the rest of the tube. At a certain drilling depth, the tube gets so long that the weight becomes greater than the desired drilling thrust. To keep the desired drilling thrust at the drill bit end of the tube, an increased pullback tension load is required at the opposite end of the drill tube. Therefore, when drilling deep, the joint needs a high tension load capacity.
A threaded joint may be modified to make it stronger. One such solution is presented in U.S. Pat. No. 5,788,401, wherein a thin walled drill tube joint with a negative thread pressure flank is disclosed. The problem is solved with a negative pressure flank of 7.5° to 15° relative to a direction perpendicular to the central axis to provide for lower stress states. Further, the threaded section of the joint tapers along the axial length of the joint, with kept thread depth. A problem with this solution is that such tube joint is difficult to manufacture.
Hence, there is a need for an improved drill tube joint.